What the gasket needs
Enough stress to flow into the flange surface roughness (seating stress), and enough stress to stay tight against internal pressure (operating stress). Too much stress crushes the gasket.
Gasket makers publish seating and maximum stress figures and constants such as the m and y factors (ASME VIII) or the EN 1591 parameters. PTFE and soft materials need lower stress and more care to avoid crush.
From stress to torque
Bolt load = gasket area x required stress (plus pressure end load). Torque is roughly T = K x d x F, where K is the nut factor (about 0.15 to 0.25 depending on lubrication), d the bolt diameter and F the bolt load. Lubricant type changes K by a factor of two, so use the specified lubricant and torque.
Tightening
- Clean flange faces. Align flanges.
- Fit new gasket centred.
- Tighten in a star pattern in at least three passes (30, 60 and 100 percent of final torque), then a circular pass.
- Re-tighten after the first temperature cycle if permitted.
- Document torque values, tool calibration and who did the job. Bolting standards such as ASME PCC-1 give a procedure.
Why joints leak
- Uneven tightening.
- Gasket creep and relaxation (PTFE, rubber).
- Flange rotation or deformation.
- Thermal cycling with differential expansion.
- Wrong gasket type for the face (raised face, flat face, ring joint).
- Damage during installation.
- Bolts stretched or corroded.
Live-loading
Disc springs under nuts keep bolt load as the gasket relaxes. Valuable for PTFE and cyclic temperatures.
Selection by duty
| Duty | Gasket |
|---|---|
| General water, low pressure | Rubber, compressed fibre |
| Strong chemicals | PTFE (filled or ePTFE) |
| High temperature or steam | Spiral wound with graphite filler, flexible graphite |
| High pressure and temperature | Ring joint metal gaskets |
| Vacuum | Metal or elastomer with fine finish |
Competence
Joint integrity is a skill. Training of fitters and verified procedures reduce leaks more than buying a different gasket.
